Process for producing molded rubber articles



Aug. 25,1942. M. CARTER 2,294,071

PROCESS FOR PRODUCING MOLDED RUBBER ARTICLES Original Filed April 30, 1936 Liquid Lnyl'ex Mold surface coaied I wil'h inert Flaked maferialr Porous mold pnrh'nliy snhmzled (20% or more) Deposifed I'Ubbl' v,

layer.

Curran! olworm dry air Reial'ive humidilg less ff-*"'- *fhan 70% a? H *VEIOtil'g in excess hiya: -of 200 feet per 3+ I minufe.

INVENTOR.

ATTORNEYS.

useful commercial articles.

Patented Aug. 25, 1942 raoouss Eon. PRODUCING MOLDED RUBBER ARTICLES Mitchell Carter, Trenton, N. .L, assignor, by mesne assignments, to Rubber Products, Inc., Chicago, Ill., a corporation of Illinois Original application April 30, 1936,- Serial No.

Divided and this application June 5.

1939, Serial No. 277,401

3 Claims.

This invention relates to the production of rubber articles from liquid latex by causing the rubber particles in the latex to be deposited on the walls of a porous mold while the water contained in the latex passes through the pores thereof.

This application is filed as a division of my copending application Serial No. 77,146, filed April It has long been known that rubber articles could be produced in this. manner, but such processes have been used commercially only to a very limited extent because of certain dimculties which have been experienced in producing Among these diillculties may be mentioned (1) the tendency of the molded article to adhere to the surfaces of the mold, causing rapid deterioration and destruction of the mold, (2) the tendency of the n rubber solids of the latex to form an impervious layer on the surface of the mold which retarded or prevented the passage of water, (3) the tendency toward the formation of air bubbles between the surface of the mold and the latex, causing a rough or honeycombed-structure in the molded article, and (4) the tendency of the molded article,' and particularly of a hollow article, to sag away from the surface of the mold,

thus deforming the article.

Itis an object of the present invention to provide a process for molding rubber articles from liquid latex in porous molds which shall be commercially practical, and which shall eliminate, among others, th'e'difliculties above mentioned.

A preferred embodiment of the invention selected for purposes of illustration is shown in the accompanying drawing, in which:

Figure 1 is a vertical section through a mold as it appears during the depositing stage of the process.

Figure 2 is a similar view as it appears during the drying stage of the process.

As far as I am aware, all prior processes for molding rubber articles from liquid latex in porous molds have used dry molds or forms and have relied upon the absorption of water into the dry porous material to build up the layer of rubber. I have discovered that this procedure is one of the causes of the dimculties which have been experlenced,for in a dry mold, the first rubber deposited tends to form an imper vious layer which retards, or in some cases, as with pure or substantially pure latex, prevents the passage of additional water, thus limiting 55 the thickness of the layer which can be formed.

- Furthermora-in a .dry mold the pores of the mold material are filled with air, and when the absorption of water begins, some of the air in the 5 pores is displaced toward the inner surface of the mold where it breaks through the surface of the article being formed, thus producing the rough or honeycombed" structure in the mold ed article referred to above.

In order to overcome this difficulty, I propose to start with a wet mold, that is, one containing a substantial quantity of the liquid which is used as the continuous phase or carrying agent for the solids to be deposited. Such, for example, as the use of water for wetting the mold when using a waterbase dispersion of rubber or so-called latex. The mold may even be saturated with water, but inasmuch as a saturated mold becomes somewhat soft and is apt to be damaged in handling, in practice I use only partially saturated molds. I find that it is satisfactory if the mold is or more saturated, but I prefer to use a mold which is approximately saturated. By saturated,'I mean a mold 2 which is so completely filled with water that it will not absorb additionalwater, and by 20% saturated, I mean a mold which contains 20% of the water which it will hold when completely saturated. 1

Shortly before the molds are used, I prepare the mold surfacesl, in order to prevent the latex from adhering thereto, by coating them with a thin layer of inert protective material. This treatment protects the mold surfaces and great- :'.5 1y prolongs the useful life of the molds. For

this purpose, I prefer to use flaked material such as powdered mica, in which the size ofthe flakes is suflicient to cover the pores of the mold and prevent the entrance of solid rubber particles into the pores of the mold, but permitting flow of liquid therethrough. The preferred method of applying the same is to prepare a suspension of mica in water, using approximately one. part mica to 20 parts water. It is desirable, however, 7 to add a small quantity of glycerine to the mix ture, approximately 5 parts of glycerine being sufficient. This mixture may be applied to the mold surfaces with a soft brush so as to secure a uniform thin layer. This method of application has the advantage that inasmuch as it is applied immediately before the mold is used, the water more or less completely saturates those portions of themold nearest to the mold surfaces,

After preparing the molds in this fashion, I

' pour in liquid latex through an opening 2 sufllcient to completely fill the mold, the mold being separated, of course, to form a layer -5 on -the inner surfaces of the mold. The presence of a common agent or ingredient, such as water, in

both the mold body and the liquid contained in the mold, assures an even and rapid withdrawal of this common component or menstruum from the body of the cast material or latex and through the body of the mold. Precautions should be taken to prevent pressure above atm0spheric pressure from being built up in the chamber, for I have found that such pressures may force moisture back through the mold wall and produce depressions on the surface of the molded articles.

I have found that a relative humidity of less than 70% should be maintained and that the velocity of the air past the molds should be in excess of 200 ft. per minute, in order to secure practical results. If a relative humidity of 50% is maintained and the velocity of the air past the molds is approximately 500 ft. per minute, these'conditipns are adequate to cause evaporation at the surface of the mold approximately equal to the maximum rate of passage of moisture through the mold wall.

As soon as a layer of rubber of the desired thickness has been accumulated, the mold is removed from the chamber and the liquid latex remaining in the mold is Poured out. The mold is then returned to the chamber until the articles are dry enough to handle or when the moisture content of 'the articles is less than 15%. Preferably the drying period should be approximately the same length as the forming period,

that is, if the forming period is one-half hour, the drying period should also be approximately one-half hour under the same conditions of temperature, humidity, air flow and pressure.

At the end of the drying period the articles are removed from the molds and are then 'further dried slowly by allowing them to stand until the moisture content is approximately 10%, The articles arethen placed in a curing chamber and are cured for whatever period may be necessary depending upon the particular vulcanizing and accelerating agents employed. In case so -called pre-vulcanized latex is used, no curing is necessary. After the article is removed from the mold, the mold is'usually in perfect condition as to moisture content for refilling for another molding operation, as it is found that they then contain more than 20% moisture. It will be understood. however, that the inner mold surfaces should be re-treated with the suspension of powdered mica previously referred to, for much of the mica previously applied will be removed when the molded article is removed. Thus the molds may ordinarily be used continuously over and over again without either drying or moistening. When so used, the articles formed therein will be uniform in thickness and weight when submitted to similar conditions for similar periods of time.

It will be understood that in the foregoing specification where I have referred to the production of articles from liquid latex, I include within ing ingredients, coloring matter and the like, and I the usual vulcanizing and accelerating agents, the mixture being maintained in liquid condition by the addition of sufiicient water to maintain a creamy consistency. I also include other dispersions of rubber in liquid. In the practice of my process I am able to produce satisfactory rubber articles from mixtures of various kinds, ranging from substantially pure liquid latex to latex which has been compounded with large quantities of compounding ingredients.

It will be understood that the invention may be variously modified and embodied within the scope of the subjoined claims.

I claim as my invention:

1. Process of treating porous molds to prepare them for the introduction'oi liquid latex for the production of molded rubber articles which comprises coating the surfaces of a porous mold with inert flaked material, the size of the flakes being suflicient to cover the pores of the mold and prevent entrance of solid particles into the pores of the mold, but permitting flow of liquid therethrough.

2. Process of treating porous molds to prepare them for the introduction of liquid latex for the production of molded rubber articles which comprises coating the surfaces of a porous mold with powdered mica flakes, the size of the flakes being suflicient to cover the pores of the mold and prevent entrance of solid particles into the pores of the mold, but permitting flow of liquid therethrough.

therethrough.

MITCHELL CARTER. 

